Experiment13: The Atom
Part 1: The hydrogen spectrum.
You will compare observed and theoretical wavelengths for the three most prominent visible
lines in hydrogen. Hydrogen in a glass tube is excited by an electric current, making it glow.
The light enters a spectroscope, which is a device containing a diffraction grating and a wavelength
scale. The grating bends different colors into different directions, making them appear at different
points on the scale. You then calculate the wavelengths from the Bohr model, and see if its
predictions match what you saw.
CAUTION: The spectrum tube operates at 5000 volts. Do not touch the posts supporting the
tube. Turn it off when not in use.
1. Put the spectroscope on books to get it the same height
as the narrow middle part of the glass tube. Check that the
opening where light enters the spectroscope isn't all the
way shut. Put the opening about an inch from the tube,
then while looking in the spectroscope, slowly turn it one
way or the other until you see bright colored lines appear.
2. There should be three bright lines, and maybe some fainter stuff you can ignore. Record the
colors and wavelengths, which are given in nanometers. If they’re too hard to see with the overhead
lights on, turn them off. If you do that, ask for a light to shine on the white part of the spectroscope
labeled "wavelength scale" on the diagram, so you can see the numbers.
3. As explained in class, the Bohr model predicts that the wavelengths ought to be given by 1/λ =
R(1/nf2 - 1/no2). Use this to calculate the wavelengths given off when an electron drops from n =
3 to 2, from 4 to 2 and from 5 to 2.
4. Compare the wavelengths you saw through the spectroscope to what you calculated.
Part 2: Radioactivity.
A) You will compare the penetrating ability of different types of
radiation, and compare the shielding ability of different materials. We
detect radiation with a Geiger tube. In the center of the tube is a wire
charged to a high electric potential. An alpha, beta, or gamma ray
entering the tube can break apart atoms of gas or atoms in the walls of
the tube, into free electrons and ions. The electrons rush toward the
charged wire, ionizing more gas atoms on the way by hitting them, so
that an avalanche of electrons crashes into the wire. The electrical pulse
so produced is counted by a box of electronics. The number of pulses
registered in some time interval measures the amount of radiation
entering the Geiger tube.
CAUTION: The radiation sources are embedded in green and orange plastic discs. They are weak
enough to be handled without special precautions, but avoid unnecessary contact: Don't play with
them. Don't put them in your pocket.
OPERATION. You have one of two different kinds of counters:
If you have a digital (off - white) counter:
1. Press power to turn it on.
2. Check that it's working correctly:
a. Press test, then stop, then reset.
b. Set count interval for one minute. Now, pressing count, and waiting one minute, should
give a reading of 3600. (When the test switch is depressed, the counter is looking at the
signal from the 60 Hz electric line, rather than the signal from the Geiger tube.)
c. If the result is incorrect, tell the instructor.
3. To take readings:
a. Check that the high voltage control is set for 400 V and press test again (The light on the
switch should go out).
b. Pressing reset then count will now count pulses from the tube for the amount of time
you've set on count interval.
If you have a non-digital (blue) counter:
1. Turn it on with the volume knob.
2. Press and hold in the red button and set the high voltage at 900 V, reading the red scale on the
meter. Watch it for a moment until it's fairly stable, then do not disturb the voltage again.
3. The response knob should be at "fast". Set the "range" knob for the most sensitive scale which
does not run the needle off scale.
4. If you turn the part of the tube housing that has three slots, the slots line up with an opening,
exposing the tube itself. Have this open.
Procedure:
1. Obtain a beta ray source (Sr-90 in a green disk).
2. Check to see if you get a stronger signal from it when the side with writing on it is up or down.
(They aren’t all the same.) Use it the way that gives a stronger signal.
3. Place it a short distance below the detector. (Blue: Clip the tube in the holder on the front of the
counter, with its window on the bottom. White: Use the second slot from the top.)
4. Record the average counting rate. (Blue: Watch the needle wave around for a couple of minutes,
and record the highest and lowest readings you see. Then, average those two numbers. White:
Make a two minute run, then divide by two to get counts per minute.)
5. Repeat step 4 with three pieces of lead stacked between the source and detector. Repeat again
with the same thickness of aluminum, and then cardboard.
6. Replace the beta source with a gamma source (Cobalt-60, orange disk) and repeat steps 1-5.
Don't use one that says "old"; that's for part (B). It doesn’t make much difference if the writing is on
top with these.
7. Conclusions:
a. Compare the penetrating ability of beta to gamma rays. (That is, for which kind of rays
does a larger fraction of the rays get through a material?)
b. Compare the effectiveness of these three materials in blocking out radiation.
B) You will determine the half life of Co-60 by comparing the activity of a newer sample to an
older one's. The apparatus is the same as in part (A). Use both disks with the manufacturer’s
labels on the bottom. The handwritten paper tags on the old ones go on top.
1. Put the gamma ray source (orange disk) as close as possible to the tube. (Blue counters: Lay the
disk on the table with the detector's window right on top of it. White counters: Use the top slot of
the tube stand.)
2. Determine an average counting rate, as before.
3. Measure the decay rate of an older sample (another orange disk, marked "old") in the same way.
Again, the printed label should be on the bottom.
This older sample is about 19 years older than the new one. Measuring its activity is a substitute
for coming back 19 years from now to measure the activity of the first sample again.
4. From the two decay rates and the time between them, calculate the half-life of Co-60. The
samples were not made very precisely; some were more radioactive than others when new. So,
which ones you happened to pick up can change your answer significantly. Expect your result to
be as much as 20% off.
In your conclusion, compare your value to the accepted one printed on the sample.
PHY 122
Report on Experiment 13: The Atom
Part 1:
color
observed λ:
Calculate predicted λ’s:
_________________ _________________
_________________ _________________
_________________ _________________
Part 2:
(A)
counts/minute:
Blue: Lowest & highest seen:
White: Reading for two minutes:
Beta source:
Nothing between: _______________ (Readings:
)
Lead: _________________________ (Readings:
)
Aluminum: _____________________ (Readings:
)
Cardboard: _____________________ (Readings:
)
Gamma source:
Nothing between: ________________ (Readings:
)
Lead: __________________________ (Readings:
)
Aluminum: ______________________ (Readings:
)
Cardboard: ______________________ (Readings:
)
(B)
Newer sample: ___________________ (Readings:
)
Older sample: ____________________ (Readings:
Calculate T1/2(Continue on back if necessary):
)